Summary - Vectors and Motion

The velocity vector points in the direction of the object's motion. Relative motion can be described by vector addition of velocities.

The acceleration vector need not point in the same direction as the object's motion. We use the word "acceleration" to describe any change in an object's velocity vector, which can be either a change in its magnitude or a change in its direction.

An important application of the vector addition of forces is the use of Newton's first law to analyze mechanical systems.